1. Field of the Invention
The present invention relates to a method for fabricating semiconductor devices. More particularly it relates to a narrow gap continuous assembly method for semiconductor devices having bump electrodes.
2. Description of the prior art:
In recent years, a highly dense assembly of semiconductor integrated circuit devices has been increasingly needed. A method for assembling devices with a narrow gap therebetween by the use of a microbump bonding process has been proposed as a method by which the number of bonding pads for devices is increased, the pitch of the pads to be bonded is minimized, and a large number of semiconductor integrated circuit devices are assembled with a minute gap therebetween (Assembly of LED arrays by a microbump bonding process; Electronic Information, Institute of Communication, Joint Society for Integrated Circuit/VLSI Design Technical Research, VLD 88-70).
Such a process will be described below by reference to FIGS. 1a to 1c.
As shown in FIG. 1a, connecting resin 23 is applied onto a first-chip fixing region of the surface of a circuit board 21 of glass on which conductors 22A and 22B are disposed. The conductors each are made of Au, ITO, or the like and the connecting resin 23 is a light setting resin such as epoxy resin, acrylic resin, or the like. Then, as shown in FIG. 1b, a first semiconductor chip 24A having gold bump electrodes 25A is disposed on the circuit board 21 in such a way that the bump electrodes 25A face the conductors 22A, respectively. Next, a pressure tool 26, which can simultaneously press two semiconductor chips against the circuit board 21, is used to press the first semiconductor chip 24A toward the circuit board 21. A portion of the connecting resin 23 that is positioned on the conductor 22A moves to the periphery of the semiconductor chip 24A so as to be resin 23A, so that the bump electrodes 25A are electrically connected to the conductors 22A respectively. Next, the ultraviolet rays 28 emitted from an optical fiber 27 radiate about half of the first semiconductor chip 24A to cure about half (the left side of the dashed line 0) of the connecting resin 23, the radiated area of the ultraviolet rays 28 being controlled depending upon the size of a lens 27A at the utmost end of the optical fiber 27. The main wavelength of the ultraviolet rays 28 is 365 nm and the illuminance thereof is about 1000 to 2000 mW/cm.sup.2. The pressure of the tool 26 is removed from the semiconductor chip 24A. The electrode 25A and the corresponding conductor 22A in the area where the resin 23 has been cured are electrically connected to each other.
Thereafter, as shown in FIG. 1c, light setting resin 23' that is the same as the light setting resin 23 is applied onto the circuit board 21 adjacent to the resin 23, and then a second semiconductor chip 24B is disposed on the board 21 in the same way as that of the chip 24A and pressed toward the board 21 by means of the pressure tool 26, the Chips 24A and 24B being spaced at a minute gap of about 5 to 20 .mu.m. Next, the ultraviolet rays 28 are radiated from the optical fiber 27 onto an area of about half of the chip 24A, a portion of the resin 23 that has not yet been cured between the chips, and about half of the chip 24B so as to cure the resins 23 and 23'. Then, the pressure of the tool 26 is released. In this way, the entire area of the chip 24A and about half the area of the chip 24B are completely electrically connected to the circuit board 21; that is, the electrodes 25A of the chip 24A and conductors 22A of the board 21, respectively, and one of the electrodes 25B and about half of the chip 24B and the conductor 22B of the board 21 in the vicinity of the chip 24A are connected to each other. In this way the chip 24A is completely fixed to the board 21 as a whole.
The above-mentioned process is repeated, as desired, a plurality of times, and a large number of semiconductor chips can be mounted on the board each with a minute gap therebetween.
When the above-mentioned conventional process is used, the inventors of the present invention have found that the following problems are unexpectedly created because the two adjacent semiconductor chips are pressed by the same pressure tool 26.
(1) When the adjacent semiconductor chips 24A and 24B are different from each other in thickness, as shown in FIG. 2, the chip 24A with a thickness that is smaller than that of the chip 24B is not pressed, causing a defective connection.
(2) When the second semiconductor chip 24B is smaller in thickness than the first Chip 24A, the first chip 24A pressed before the second chip 24B is pressed, so that, as shown in FIG. 3, the connecting resin squeezed out from the periphery of the first chip 24A flows toward the second chip 24B, which causes the second chip 24B to shift from a given position to a position that is apart from that of the first chip 24A.
As mentioned above, when a large number of chips are tried to be assembled on a circuit board with a minute gap therebetween by the conventional microbump bonding process, the above-mentioned problems are created, which leads to a defective assembly.